1. Field of the Invention
This invention relates generally to the dispersing of fluidized solids into a vessel. More specifically, this invention relates to a method and apparatus for distributing a stream of spent fluidized cracking catalyst particles into a regenerator for carbon removal.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in a contacting zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the contacting zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high boiling point hydrocarbons to lighter boiling hydrocarbons in the heating oil or gasoline (or lighter) range. In the FCC process, hydrocarbon feed is contacted in one or more reaction zones with a particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons. The heavy hydrocarbons in the feed crack to lighter hydrocarbons. During cracking carbonaceous hydrocarbons or xe2x80x9ccokexe2x80x9d deposit on the catalyst to yield xe2x80x9ccokedxe2x80x9d or xe2x80x9cspentxe2x80x9d catalyst. The cracked products are then separated from the coked catalyst. The coked catalyst is then stripped of volatiles, usually by steam, and then is regenerated in a catalyst regenerator. In the regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Flue gas formed by burning the coke in the regenerator may be treated for removal of particulates and conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Emphasis on the environmental importance of reduced NOx formation in flue gas has prompted much work in various areas. NOx, or oxides of nitrogen, comes mainly from the oxidation of nitrogen compounds in the hydrocarbon feed, with perhaps some slight additional nitrogen fixation, or conversion to NOx of nitrogen in regenerator air.
Although all FCC regenerators produce some NOx, the problem is more severe in bubbling bed regenerators, as opposed to high efficiency regenerators. High efficiency regenerators burn most of the coke in a fast-fluidized bed coke combustor. Such regenerators have few poorly fluidized regions. Bubbling bed regenerators may have poorly fluidized regions and will have large bubbles of air passing through the bed, leading to localized areas of high oxygen concentration. Although the reasons for the different NOx emissions in these two types of regenerators are perhaps not completely understood, all agree that NOx emissions are usually significantly higher, frequently twice as high, from bubbling bed regenerators.
One area of work on NOx reduction pertains to flue gas treatment methods that are isolated from the FCC process unit. With flue gas treatment, it is known to react NOx in flue gas with NH3. NH3 is a selective reducing agent, which does not react rapidly with the excess oxygen, which may be present in the flue gas. Two types of NH3 processes have evolvedxe2x80x94thermal and catalytic. Thermal processes, such as the Exxon Thermal DENOX process, generally operate as homogeneous gas-phase processes at very high temperatures, typically around 840xc2x0 to 1040xc2x0 C. The catalytic systems that have been developed operate at much lower temperatures, typically at 150xc2x0 to 450xc2x0 C. These temperatures are typical of flue gas streams. Unfortunately, the catalysts used in these processes are readily fouled, or the process equipment plugged, by catalyst fines that are an integral part of FCC regenerator flue gas. U.S. Pat. No. 521,389 and U.S. Pat. No. 434,147 disclose adding NH3 to NOx-containing flue gas to catalytically reduce the NOx to nitrogen. U.S. Pat. No. 5,015,362 taught reducing NOx emissions by contacting flue gas with sponge coke or coal, and a catalyst effective for promoting reduction of NOx in the presence of such carbonaceous substances.
Flue gas treatment methods are effective, but the capital and operating costs are high. Therefore, the alternative areas within the FCC process unit itself should be examined, which include feed treatment, catalytic approaches, and process approaches.
First, some refiners now go to the expense of hydrotreating feed. This is usually done more to meet sulfur specifications in various cracked products, or a SOx limitation in regenerator flue gas rather than a NOx limitation. Hydrotreating will reduce to some extent the nitrogen compounds in FCC feed, and this will help reduce the NOx emissions from the regenerator. Again, there is typically a high cost for this procedure and it can usually only be justified for sulfur removal.
Second, there are catalytic approaches to NOx control. These approaches are generally directed at special catalysts which promote CO afterburning, but which do not promote formation of as much NOx. U.S. Pat. Nos. 4,300,997 and 4,350,615 are both directed to use of a Pdxe2x80x94Ru CO-combustion promoter. The bimetallic CO combustion promoter is reported to do an adequate job of converting CO to CO2, while minimizing the formation of NOx. U.S. Pat. No. 4,199,435 suggests steam treating a conventional metallic CO combustion promoter to decrease NOx formation without impairing too much the CO combustion activity of the promoter. U.S. Pat. No. 4,235,704 indicates too much CO combustion promoter causes NOx formation, and calls for monitoring the NOx content of the flue gases, and adjusting the concentration of CO combustion promoter in the regenerator based on the amount of NOx in the flue gas. As an alternative to adding less CO combustion promoter, the patent suggests deactivating it in place, by adding something to deactivate the Pt, such as lead, antimony, arsenic, tin or bismuth. U.S. Pat. No. 5,002,654 taught the effectiveness of a zinc-based additive in reducing NOx. Relatively small amounts of zinc oxides impregnated on a separate support having little or no cracking activity produced an additive which could circulate with the FCC equilibrium catalyst and reduce NOx emissions from FCC regenerators. U.S. Pat. No. 4,988,432 taught the effectiveness of an antimony-based additive at reducing NOx. 
However, many refiners are reluctant to add additional metals to their FCC units out of environmental concerns. One concern is that some additives, such as zinc, may vaporize under some conditions experienced in FCC units. Many refiners are concerned about adding antimony to their FCC catalyst inventory. Such additives would also add to the cost of the FCC process and would dilute the FCC equilibrium catalyst to some extent.
Thirdly and finally, there are process approaches. Process modifications are suggested in U.S. Pat. Nos. 4,413,573 and 4,325,833 directed to two-and three-stage FCC regenerators, which reduce NOx emissions. U.S. Pat. No. 4,313,848 teaches countercurrent regeneration of spent FCC catalyst, without backmixing, to minimize NOx emissions. U.S. Pat. No. 4,309,309 teaches adding a vaporizable fuel to the upper portion of a FCC regenerator to minimize NOx emissions. Oxides of nitrogen formed in the lower portion of the regenerator are reduced in the reducing atmosphere generated by burning fuel in the upper portion of the regenerator. U.S. Pat. No. 4,542,114 minimized the volume of flue gas by using oxygen rather than air in the FCC regenerator, with consequent reduction in the amount of flue gas produced.
In U.S. Pat. No. 4,828,680, NOx emissions from a FCC unit were reduced by adding sponge coke or coal to the circulating inventory of cracking catalyst. The carbonaceous particles selectively absorbed metal contaminants in the feed and reduced NOx emissions in certain instances. Many refiners are reluctant to add coal or coke to their FCC units; such carbonaceous materials will burn and increase the heat release in the regenerator. Most refiners would prefer to reduce, rather than increase, heat release in their regenerators. U.S. Pat. No. 4,991,521 showed that a regenerator could be designed so that coke on spent FCC catalyst could be used to reduce NOx emissions from an FCC regenerator. The patent taught the use of a two stage FCC regenerator. Flue gas from a second regenerator stage contacted coked catalyst in a first stage. Although effective at reducing NOx emissions, this approach is not readily adaptable to existing units. Another use of coke on spent catalyst to reduce NOx was reported in U.S. Pat. No. 5,006,495. The incoming spent catalyst, or at least a portion of it, was added to the dilute phase region of a bubbling bed regenerator, so that the coke on catalyst could reduce NOx species in the dilute phase flue gas. This approach is interesting, but may increase dilute phase catalyst loading, and would require considerable unit modification.
We have found a simple, direct, and economical solution in that the apparatus and method of distributing catalyst into the regeneration vessel can dramatically affect the quality of the flue gas emissions produced upon coke combustion.
Previous art regarding catalyst distribution has focused on improved catalyst mixing in the regenerator to provide more complete and efficient catalyst regeneration. Better distribution and mixing also avoid dilute phase CO combustion or afterburning in the offgas. U.S. Pat. No. 5,773,378 disclosed a spent catalyst distributor apparatus and retrofit method to radially discharge spent catalyst and 10-50% of the regeneration air into the dense phase of the catalyst. U.S. Pat. No. 5,635,140 showed a self-aerating spent catalyst distributor to discharge catalyst radially and downwardly from a centerwell via lipped trough arms into the catalyst bed. U.S. Pat. No. 4,150,090 disclosed a similar system but included an aeration means in the trough arms to assist in fluidization and expulsion from the troughs.
An article disclosed an extension to a spent catalyst standpipe that directs catalyst into a fluidized trough located below the catalyst bed level. Joseph Wilson and Chris Ross, FCC Revamp Improves Operations at Australian Refinery, OIL and GAS JOURNAL, Oct. 25, 1999, at 63.
U.S. Pat. No. 4,615,992 disclosed a process for regeneration which included a horizontally placed baffle located below the catalyst bed level, and a concentric well pipe extending around a vertical standpipe.
U.S. Pat. No. 5,156,817 disclosed additional devices for discharging catalyst admixed with gas into a regeneration bed.
It is an object of this invention to provide a method and apparatus that simplifies the reducing or eliminating of non-uniformity in the delivery of particles into a fluidized particle bed within a vessel.
It is a further object of this invention to provide an apparatus and method for distributing spent catalyst uniformly onto a catalyst bed within a fluidized regenerator.
It is a further object of this invention to provide a method and apparatus that is susceptible to simple repair, replacement, or modification that provides a well dispersed spent catalyst layer across the top of a catalyst dense bed within a fluidized catalytic cracking regenerator.
It is a further object of this invention that the spent catalyst delivered to the top of the regenerator dense bed provide a curtain of coke that acts to reduce NOx to N2 and CO2.
The objects of this invention are achieved by a specific form of a catalyst distributor arrangement that places coked catalyst horizontally into regenerator and onto the surface of the dense phase bed. The surface of the catalyst bed is considered to be within the upper and lower fluctuations of the transition boundary from a dense fluidized catalyst phase to a dilute flue gas phase with entrained catalyst. A hydraulic head of accumulated catalyst in a fluidized hopper vessel acts to provide the driving force for catalyst transport and flow. Dispersion onto a catalyst bed takes place through a header connected with multiple outlet arms. An aeration means can assist flow within the header by providing additional fluidization gas.
Other mechanical and operational advantages can result from the incorporation of this invention. Such advantages include FCC unit debottlenecking. Since the delivery of spent catalyst is more uniform, it is possible to contain more CO burning within the catalyst bed. This reduces the amount of afterburn in the dilute phase which often limits the effectiveness or capacity of many FCC units. This allows oxygen to be used more effectively, thus increasing the coke burning capacity at the same air flow rate. An additional mechanical advantage is the ease of installation into existing regeneration vessels, which allows revamps to be accomplished within a typical existing unit turnaround schedule.
Accordingly, in one embodiment, this invention is a method of regenerating FCC catalyst in a regenerator having a spent catalyst inlet for receiving spent catalyst from a stripper and an air distribution system at a lower end of the regenerator; wherein the method comprises the following steps. First, collecting catalyst from the spent catalyst inlet in a hopper and fluidizing the collected catalyst to provide a hydraulic head to assist catalyst flow. The next step is passing the catalyst to multiple points near a surface of a dense phase catalyst bed using a horizontally extended header having a plurality of horizontally extended outlet arms. Catalyst may also be passed through an opening in the hopper to a point near the surface of the catalyst bed. The next step is contacting the catalyst with fluidization gas in the regenerator to burn off at least part of the coke present on the spent catalyst. Finally, a regenerated catalyst is produced which then can be recovered from a dense phase of the catalyst bed. In preferred embodiments the hopper is fluidized with an air distributor located at the bottom of the hopper, and the header is fluidized with a means for aeration such as an aeration lance inserted into the header to further assist catalyst flow. The fluidization gas preferably comprises air. The top of the hopper is open to the regenerator. The method further preferably includes the step of producing a recovered off gas, or regenerator flue gas, containing reduced NOx as a result of the improved catalyst distribution.
In an apparatus embodiment, this invention has a hopper, an air distributor located at the bottom of the hopper, and a horizontally extended header having a plurality of horizontally extended outlet arms for placing catalyst near the top surface of the catalyst bed within a FCC regenerator. The header is in communication with the hopper. When this apparatus is installed in a regenerator having a spent catalyst standpipe, the hopper is in communication with the spent catalyst standpipe and the header is fixed with respect to the wall of the regenerator. The hopper also contains an outlet on its side in order to pass a portion of the catalyst near the top surface of the bed. In preferred embodiments, the header further comprises a means for aeration, which can be further characterized as an aeration lance with a plurality of orifices. The hopper is open at the top to provide an alternate contingency means for catalyst transport into the regenerator. The outlet arms may be arranged at various angles and places on the header, with a preferred angle range being 30 to 150 degrees, and an especially preferred angle range being 55 to 100 degrees. Drains may also be placed in the header to permit additional alternative pathways for catalyst flow to the regenerator.
Additional objects, embodiments, and details of this invention can be obtained from the following xe2x80x9cdetailed descriptionxe2x80x9d.